1. Field of the Invention
This invention relates to building materials and methods for making the same, and more particularly to the addition of low density additives (LDA) into cementitious cellulose fiber-reinforced building materials.
2. Description of the Related Art
Fiber-reinforced cement (FRC) products such as water-resistant building sheets have been used for building since 1895. In recent history reinforcing fibers used in such products have included not only asbestos fibers, but also cellulose fibers (see Australian Patent No. 515151), metal fibers, glass fibers and other natural and synthetic fibers. Typically, the density of such building sheets is from about 1.2-1.7 g/cm3, the variation in density typically being achievable by compression and dewatering of the fiber cement slurries used in manufacture and by varying the amount of fiber used. At these densities, the cement based matrix has few voids, which results in lower water absorption which has usually been considered necessary for good durability performance of cement matrices.
The densities of fiber cement described above mean the products are heavier than timber based products of equal dimension and have reduced workability. Workability encompasses the ease with which a board is handled and installed. Therefore, fiber cement building products are more difficult to cut, machine and nail than timber and timber based products. In this regard, the density of natural timber sheets typically ranges from about 0.7-0.9 g/cm3 for dry hardwoods and from about 0.38-0.6 g/cm3 for dry softwoods. Thus, a density-modified fiber cement material with density similar to timber may be expected to improve workability and enable lighter, more nailable, easier to cut and easier to machine products to be manufactured. However, this would have to be achieved while retaining the durability, fire resistant, rot proof and water resistant properties of fiber cement if the density modified fiber cement is to be used in the same range of applications.
Prior art describes how lightweight inorganic powders can be added as density modifiers in cement or fiber-reinforced cement materials. Low density additives for FRC products are defined as having a loose bulk density of about 0.8 g/cm3 (about 50 lbs./cu.ft.) or less. The typical low density additives (LDA) used include low bulk density calcium silicate hydrates (CSH), expanded polystyrene beads (EPS), expanded vermiculite, expanded perlite, expanded shale, and expanded clay. The density modification of cement-based materials with such inorganic particles is primarily achieved by introducing porosity into the material. Typically, the pore spaces are filled with water when the material is submerged in water or exposed to rain for a length of time. This causes these materials to have poorer wet to dry dimensional stability (moisture resistance), a higher saturated mass, and poor freeze-thaw resistance.
Accordingly, there is a need for a lightweight FRC building material and method for manufacturing the same with improved wet to dry dimensional stability over that of typical density modified products. Secondly, the lightweight building material should maintain similar wet to dry dimensional stability as that of FRC products without density modifiers if the density modified material is to be used in the same range of applications. In addition, it is highly preferred in some applications that the material also have a low saturated mass, good freeze-thaw resistance, and high temperature dimensional stability. Finally, it is also desirable to have a FRC building product where lower ranges of densities closer to that of timber and timber based products can be achieved with improved durability.